It has already been proposed to provide tunnel-excavating machines which are formed with cutting means adapted to be advanced relative to a pair of support frames which can be provided with means bearing against the tunnel wall.
A tunnel-excavating machine of this type generally comprises a pair of support members which are urged outwardly on the forward and rearward support frames independently to support the frames alternately against the wall.
The support means of one of the frames can thus be urged outwardly into engagement with the tunnel wall while the excavating head is forced forwardly, the other frame, whose support members are drawn inwardly, being simultaneously advanced. During the next stage of operation, the members of this latter support frame are urged outwardly to engage the tunnel wall while the support members of the first-mentioned frame are retracted and the latter support frame is then advanced. The process can then be repeated over the entire length of the stretch of tunnel to be excavated.
While such tunnel-excavating machines have a relatively large rate of advance through subterranean strata, the use thereof involves certain difficulties. In order to provide a fairly fixed support for the cutting head, the support members of the frame which is fixed during advance of the head, must be urged outwardly at extremely high pressure. When the strength of the strata through which the tunnel is to be excavated is limited, such high supporting pressures, applied over the limited axial length of the support frame and its members, have been found to disrupt the geological stratum through which the machine is advanced.
Damage to the subterranean formation reduces the speed with which the machine can excavate and, if insufficient support is provided, prevents the cutting head from advancing properly.
Furthermore, when the wall of the tunnel is damaged in this manner, there is a danger to operating personnel behind the machine and effective stabilization of the completed tunnel wall is difficult. In fact, the use of such machines in strata which have already been subjected to geological faults and the like, has been found to be impractical.
There have also been provided so-called cutter excavators in which an annular blade shield having a plurality of blade segments or members is advanced through a subterranean stratum. Such cutter blade machines, in which the leading edge of each of the shield segments forms a cutter which actually cores the tunnel from the stratum, have been found to be effective only for relatively soft and yieldable subterranean structures. The machines are not usable where varying geological conditions obtain or for harder subterranean formations.